1. Field of the Invention
The present invention relates to a self-diagnosis method for a mass flow controller. In particular, the present invention relates to a self-diagnosis method for a mass flow controller, which makes it possible to diagnose the state of change in gas flow rate in the mass flow controller while being assembled in the system.
2. Description of the Related Art
For example, the apparatus for producing thin films and the apparatus for dry etching, which are used in the process for producing semiconductors, require precise control of the flow rate of the process gas fed to the process equipment. Therefore, a mass flow controller is inserted into the pipework for supplying the process gas to the process equipment in order to control the flow rate of the process gas. The gas flow rate is set by adjusting the control voltage applied to the control valve of the mass flow controller.
However, the mass flow controller is equipped in order to control the gas flow rate. Therefore, the mass flow controller includes thin tube components such as an orifice disposed at its inside. When the process gas is, for example, a gas which causes deposition of solid matters, the solid matters deposit in the pipework or in the mass flow controller along with the passage of time as the process gas passes therethrough. As a result, a situation occurs, in which the effective cross-sectional area of the pipework in the mass flow controller is substantially decreased.
In such a situation, any change occurs in the relationship between the control voltage applied to the mass flow controller and the gas flow rate corresponding to the control voltage. The flow rate of the gas passing through the mass flow controller changes even when the control voltage is not altered. Consequently, the state of the process gas undergoes occurrence of change.
Further, if the control is continued in the state in which the effective cross-sectional area of the pipework is decreased in the mass flow controller, particles which result from the solid matters flow out to the process equipment together with the process gas. As a result, any inconvenience arises in the process equipment.
Therefore, it is desired to measure the flow rate of the mass flow controller. A system is known to examine the absolute flow rate of the mass flow controller as disclosed, for example, in Japanese Laid-Open Patent Publication No. 7-281760. That is, when the absolute flow rate of the mass flow controller is examined, the process gas is introduced from a process gas supply source via a shut off valve to a measuring tank. The process gas, which has been introduced into the measuring tank, is supplied to the process equipment via the mass flow controller. A pressure sensor is provided to measure the pressure at an input end of the mass flow controller. The introduction of the process gas into the measuring tank is shut off by closing the shut off valve. The process gas, which has been accumulated in the measuring tank, is allowed to flow to the mass flow controller. Simultaneously, the pressure sensor is used to detect the pressure on the inflow side of the mass flow controller to measure the period of time until the detection pressure detected by the pressure sensor is lowered to a predetermined value. The absolute flow rate of the mass flow controller is examined on the basis of the measured time and the volume of the measuring tank.
However, when the measurement is performed in accordance with the conventional examination method as described above, the following problem arises. That is, in addition to the mass flow control system arranged for the ordinary operation, it is necessary to equip, for example, the measuring tank, the pressure sensor, and the shut off valve to perform the examination.